Multileaf collimators: modern beam shaping

One of the most important aspects of therapeutic radiation delivery is beam shaping. The most common technology currently used to shape x-ray beams is the multileaf collimator or MLC.

120 leaf multileaf collimator.

MLC's consist of dozens of independently moving slats, or leaves, which can collimate the beam into nearly arbitrary shapes. MLC's were first introduced in the 1960's in Japan as a means of replacing conventional blocks in 3D conformal radiation therapy (3DCRT). Today they are standard on most therapy linacs and enable modern techniques based on intensity modulation.

Key advantages of MLC's include:

• Dynamic movement of leaves during delivery allows for intensity modulated fluence patterns not achievable with conventional blocks.
• Finite set of possible aperture shapes results in a reasonable solution space for inverse planning optimization for IMRT, VMAT, etc.
• Significantly more convenient than cutting custom blocks for every field.

Drawbacks of MLC's include:

• Large number of leaves increases possibility of mechanical failure (i.e. reliability issues).
• Non-zero inter-leaf leakage.
• Smoothness of shaping dependent on size of leaves and speed of leaf motors.

Almost everyone can agree that MLC's have been a huge advance in radiation therapy, with the advantages far outweighing the disadvantages. I plan to discuss MLC alternatives in a future post.


Image courtesy of Varian Medical Systems, Inc. All rights reserved. (source)

The basics of IMRT: an overview

Intensity modulated radiation therapy (IMRT) is one of the workhorse delivery methods in current radiation therapy. In many ways, it is a significant step up over the previous standard technique of 3D conformal radiation therapy (3DCRT). I'm going to start talking about IMRT with a very basic overview of how it works.

IMRT is a technique to plan and deliver MeV range x-ray therapy. The main advance with IMRT over 3DCRT is the algorithmic optimization of dose from all delivery angles at the same time to meet a pre-defined set of objectives, so-called inverse planning. This is accomplished by using multileaf collimators, which can create arbitrary aperture shapes (within reason), thus modulating the dose to the targets. In turn, this gives the dose optimization algorithm a large number of parameters to work with to achieve the desired dose shape.


Key steps in IMRT planning and delivery:

• Acquisition of patient geometry (via CT, MRI, etc).
• Delineation of targets and avoidance volumes.
• Beam angle and energy selection.
• Optimization of fluences to desired prescription and avoidance objectives.
• Assessment of DVH's.
• QA via secondary calculations and field measurements.

Diagram of a multileaf collimator.

Further reading:

• Bortfeld, IMRT: a review and preview, Phys. Med. Biol. 51 R363, 2006 doi:10.1088/0031-9155/51/13/R21
• KY Cheung, Intensity modulated radiotherapy: advantages, limitations and future developments, Biomed Imaging Interv J 2006;2(1):e19 (open access)

Top image by Taheri-Kadkhoda et al. Radiation Oncology 2008 3:4 doi:10.1186/1748-717X-3-4 licensed under CC licensing.
Bottom image by ZEEs and licensed under CC licensing.

Measurement theory in the clinic

Collecting data is one of the main day-to-day activities of the clinical physicist. Examples from the radiation therapy clinic include PDD's, mechanical alignment parameters, ambient temperature and pressure, source autoradiographs, IMRT planar dose distributions, setup SSD's, and CT number data. Patient outcomes, safety, and a host of other issues often rely on the quality and interpretation of that data.

One thing that sets medical physicists apart from most everyone else in the clinical environment and is crucial to performing our jobs well is having a strong grasp on the theory of measurement. Key concepts include: the statistical nature of data, the role of calibration, instrument resolution, and uncertainty and error propagation.

While we may not determine strict error levels for every measurement we make, it's important to have a grasp of these concepts while collecting and interpreting data.

• Why does my IMRT data taken with a planar detector array pass, even though it "looks" totally different than the TPS generated data? Probably because the detector array has relatively low resolution and the dose shape you see is the result of interpolation by software.
• Is my data skewed due to systematic or random errors?
• Is my new outlier subject to regression to the mean or the start of a new trend?
• Are my data groupings producing a Simpson's paradox?

What are your thoughts?

What is medical physics?

...or this blog has to start somewhere.

If you are reading this blog, you are likely familiar with medical physics, but for my first official post I'm going to talk about what medical physics is.

Medical physics is a field of applied science and engineering, in which physics-based techniques form the basis of diagnostic and therapeutic medical technologies. The most well known of these technologies are x-ray imaging, CT, MRI, ultrasound, and radiation therapy. Many medical physics technologies utilize ionizing radiation. The potential hazard of ionizing radiation is arguably the reason why medical physicists exist as clinical personnel, versus solely as researchers and developers, as is the case with biomedical engineers.

While most broadly medical physics is the application of physics techniques across all of medicine, the term "medical physics" is generally used to refer to three primary areas: diagnostic imaging, nuclear medicine (radionuclide based imaging and therapies), and radiation therapy. The majority of clinical medical physicists work in radiation therapy.

Historically medical physics arose from the application of physics discoveries and technologies to medicine, most importantly the x-ray for imaging starting in 1896. As these technologies were more broadly adopted by hospitals, more physicists and knowledgeable personnel were needed in clinical settings. Eventually, medical physics specific training emerged and technologies, such as medical electron linear accelerators, were developed from the ground up with medicine in mind.

Today medical physicists work as clinicians, academic researchers, industry experts, and educators, or spend their time as any combination of those.

Further reading:

• Wikipedia entry on Medical Physics
• What do Medical Physicists Do? (AAPM)

365 Days of Medical Physics blogging

Welcome to my new blog! I'm intending to make short posts (nearly) every day for the next year covering a broad range of medical physics topics.

The idea behind this blog is largely to help motivate me to consistently push my medical physics knowledge, but also to foster discussion online relevant to others learning medical physics.

I am a medical physicist involved in radiation therapy, so this blog will center on issues related to clinical radiation therapy physics, but I will also venture into some diagnostic and research territory. For topics more closely related to my research on particle therapy and computational medical physics, I will probably post on Will Work for Science, the blog I co-author will Herr Dr. Niels Bassler.

I hope you enjoy the posts. Please comment, tell me how awesome this is, correct me, and/or make topic suggestions!